This question differs slightly from the many excellent threads on topics similar to this one. In the fictional universe I am contemplating, the same laws of physics apply as in ours, but because of a slightly different evolution, the make-up of the universe is a bit different. Also, for some physical laws which are presently up in the air as of this writing, I can choose any solution that does not contradict the known laws of physics. (So no magic.) For example, in this fictional universe there are a bunch of primordial black holes around on their last legs (very small,about to explode in a shower of Hawking radiation), and there are larger concentrations of antimatter in free space than in ours. Assuming there was a way to detect where appropriate black holes were and to distinguish the antimatter from ordinary matter (without a lot of probes), could either one of them (or both) be used as an energy source for an intergalactic ship? (By the way, yes I know I could also simply construct my universe such that there was a lot more intergalactic hydrogen and then use some variation on the Bussard ramjet, but there is a reason I want to incorporate the primordial black holes and antimatter into the story.)
A propulsion system based entirely on the harnessing of gravity along the lines of the Space Coupe in Dick Tracy.
Our 'Laws pf Physics' are all based on assumptions and constraints we have made, and then we force our laws to fit these assumptions and constraints.
Suppose we based everything on 'G' and not 'c'?
For instance, all of Einstein's theories associated with relativity are based on 'c' being the founding universal constant. His 'thought experiments' were to illustrate that the 'speed of light' (actually, 'c' is not the 'speed of light', it is just a constant with the units 'distance/time') was constant across all 'relativistic frameworks', and the factors that determine the 'relativistic framework' are based on concepts of velocity, acceleration, and time. What would the theories look like if he had used 'G' as the universal constant? That is, based all of our cosmological theories on the concept that 'G' is absolutely constant no matter what the speed or velocity? But then, 'G' is completely unrelated to 'time', so 'speed and velocity' have no bearing on it. They can not be the basis for a 'relativistic framework' for 'G'. The constant 'G' is all about force, distance, and mass, so those terms would have to define the 'relativistic framework' (note the absence of 'time'.)
Suppose the 'relativistic framework' was determined by gravity ('G') and force, distance (space), and mass, not electromagnetic propagation ('c'), distance and time (speed and velocity)? Would not all of our 'modern' physics be based around gravity, instead of electromagnetic propagation? Would we not currently know much more about gravity than we now know? Perhaps we would not even be so hung up on our obsession with 'time' (since it has no influence on 'G')? Einstein all but ignored gravity and the gravitational constant, somehow arguing that it was some artifact of space/time warping. Imagine the reverse - a construct of physics based on gravity ('G') as the basis of force-distance/mass instead of 'c' for space/time, and assuming 'c' and time were an artifact of 'G'?
m/s vs Nm^2/kg^2
Side bar, from the equation e=mc^2, the units of 'c' are derived to be m/s, distance/time, giving us the basis for describing the universe in terms of space/time. The units for 'G' are force-distance/mass, no unit of time involved, a completely different concept of the universe.
The evolution of our mind determined how we constructed our Laws pf Physics
Looked at another way, our Laws of Physics are not, in fact, unrelated to our own evolution. A major factor in our evolution has been our proselytization about the 'passage of time', making 'time' a very important factor in our evolution of the universe as having or being a linear, cause-effect, sequential nature. What if humans, and human knowledge, evolved in some different direction? Suppose a species developed a completely different concept of 'time', as an artifact and not as something that drives physics? All of our science is based, in fact, on the principles of cause-effect relationship and mathematics. Our 'equals' sign in math is understood to be 'becomes' instead of 'is the same as'. Suppose a civilization had evolved based on a non-sequential wholistic thinking and mathematical system? Sort of like an entire knowledge system based on pure non-analytical reasoning and geometry? Of pattern recognition, instead of breaking everything down into individual constituent elements and analysing them individually? A way of thinking in which the 'equals' sign always means 'is the same as' and is simultaneous, not linear? The fact that we have very little concept of this thinking style in our current physics paradigm does not invalidate that a species could have evolved along this path, and would therefore have very different concepts towards 'Laws of Physics' than we do.
Is our physics limited by our ideological abhorrence to 'Spooky action at a distance'?
In fact, our current abhorrence to accepting 'spooky action at a distance' has essentially locked us out of a tremendous amount of 'scientific discovery'. Our inherent scientific bias to make everything time-dependent and sequential has hamstrung any efforts to expand our 'Laws of Physics' into the wholistic milieu. We have become obsessed with time, the passage of time, and developing a textbook of physics based on time.
So, given that we know so little about gravity, and 'G', and our 'laws of Physics' are all constrained by our evolutionary predisposition to thinking in terms of linear 'cause and effect' instead of 'simultaneous probability' and the probabilistic nature of the universe, I can envision a species that had evolved differently than us, and whose brain structures were less linear (left-brained) and more wholistic (right brained). I can foresee that they would have a very different concept and perspective on 'Laws of Physics', unconstrained by our limitations and 'thinking style'.
Yes, this is perhaps much more a philosophical approach to your question than a 'physics based' approach, but really, isn't that what we are talking about when you investigate a civilization that had evolved completely different than we have, in all respects?
Would gravity not be their focus of cosmology?
Their 'constraints' and 'assumptions' about the 'nature of physics' would be very different from ours, and they could very easily be imagined to have principles of physics that we, because of the limitations of our evolutionary thinking that constrain us (particularly 'time'), have imposed on the universe. Their physics textbook might look very different than ours. Especially considering the environment their thinking and knowledge base evolved in. It would be natural for them to be much more concerned with a study of gravity and building a cosmological physics of gravity than of electromagnetic radiation. 'Spooky action at a distance' takes on a whole new perspective when that 'spooky action' is caused by the gravity of black holes, and is not really so 'distant' and not nearly as 'ignorable'.
So, a civilization that has evolved with a very different approach to thinking and to structuring the universe as they are living in and perceiving it, could easily be envisioned to have discovered a drive based on studying and mastering the principles of gravity, and the ability to focus and concentrate gravitational effects (waves? field? Boson?). With those 'black holes', there would be an abundance of 'fuel' for such a propulsion system, and a much greater compelling reason for them to study and harness it. They could, perhaps, investigate the 'laws pf physics' and the universe from a probabilistic and not a deterministic viewpoint, and consider quantum probabilistic non-time-dependent wholistic ideas of field theories and universal simultaneity instead of being hung up on time-dependent electromagnetic wave propagation and the concepts of time, speed, distance, velocity, and such.
Edited g to G